Reliability in semiconductor circuits is an important aspect to chip design, especially with the increasing complexity of the circuits and the increased density of the silicon on which the circuits reside. Connections to inputs, outputs, and power are susceptible to ESD events that can damage internal components. Fundamentally, ESD is a short discharge of electric energy caused by the sudden release of an electrostatic build-up of electrical charge. If ESD currents flow suddenly through electronic components, the high currents can literally melt the carefully formed layers of an Integrated Circuit (“IC”) device.
Modern semiconductor devices have increasingly small features and complex circuits having many additional interface pins and may further comprise multiple power pins and even multiple voltage levels, all of which further increase the susceptibility of the circuits to ESD events.
Circuits of IC devices are typically very susceptible to damage if ESD events cause input voltages that are outside of the power supply lines for the circuits. ESD protection circuits therefore have often included diodes to shunt such outlying input voltages back to the power supply circuits, which accordingly prevents high currents from flowing through what would otherwise be improperly biased transistors and other circuit elements in the ICs. For example, a simple diode having its anode at a signal input and its cathode at a positive power supply voltage or “rail” will conduct current once the input has exceeded the power supply voltage rail by the diode's turn-on voltage, which in a typical PN junction is approximately 0.7 volts. Relative to the ground supply voltage, that same input might have the cathode of another diode at the input and the anode of the diode at the ground supply rail, in which case the diode will begin to conduct if the signal input falls below the ground supply rail by more than the diode's turn-on voltage.
During normal circuit operation, when the signal inputs are within the power supply rail voltages, the diodes are off and ideally have no effect on the circuit operation. To further ensure that the ESD protection diodes have no effect during normal circuit operation, it is often desirable to provide diode circuits having higher turn-on voltages relative to the supply rails. In a series connection of discrete (i.e., non-IC) diodes, the diode turn-on voltages are additive, such that two diodes in series will have a turn-on voltage of approximately 1.4 volts. In a practical implementation using integrated circuit or transistor diodes, however, the stacking of PN junctions form parasitic bipolar transistors. The parasitic transistors allow current to sink to the substrate, which increases leakage current; and, due to this leakage current, the addition of diodes does not necessarily linearly increase the turn-on voltage from the diode string. The result is that still more diodes are needed to support an increased voltage.